tech issue: dash light christmas tree

Thank you
Today (not because of the forum) is a day I am way too likely to blow my top... so appreciate you holding his hand..

 

plot thickens....cruise control system turns on, but won't set. I'll still go through the ECU test and get a detailed code scanner, but this does lend itself to being a rear wheel sensor issue from what I've read. agree? disagree?

 

Geez, dude. No. Could it be? Sure. Is it the likely culprit given all the information? No. Will a good scan tool precisely point you to a wheel speed sensor? Absolutely, without question, yes.

Whatever is causing your ROOT issue is disabling all these systems. When the TRAC system, VSC system, ABS system, etc. is all disabled, cruise control ain't 'gon work. Zero surprise here, could've already told you that.

What's interesting about your issue??? THE REAR DIFF LOCK LIGHT FLASHING. It sticks out like a sore thumb because most malfunctions that light the VSC, TRAC, ABS, etc. lights do NOT include a flashing diff lock light. Diff lock fits the best, it's specifically unique to your case. Since it's the best bit of information in your situation, GO THERE FIRST. Then if that doesn't work out, do the regular trouble shooting for the remaining lights which is a myriad of things that is a never ending rabbit hole without a good scan tool.

 

right, as I said I would still do the ECU test (disabling all systems) first. Was just providing additional info as it arose, sorry that upset you so much.

 

The rear diff lock light doesn't flash just for fun.

 

I on the other hand, do flash just for fun.

 

New code reader came today, did a scan, here are the codes which I’m currently researching.

 

Finally had the time to take the dash apart a bit to get to the 4wd ECU. They sure put it in a fun place to get to....

Anyway, I unplugged both the long black and shorter white plug from the ECU. When I turned the truck on, the diff lock light is now off, but the ABS and TRAC lights are still on. Later tonight or tomorrow I'll being voltage testing. My Q is now which plug gets the positive vs negative ends of the multimeter or doesn't it matter. Also, do I test blue to blue? then black to black? I am testing for continuity, or Ohms at the 10 amp level.

I also found this diagnostic procedure. If I'm reading it right, the first step is to connect the multimeter to 1 and 5 sockets on the smaller white plug, maybe leaving the longer black one plugged in? that part is not clear.

http://www.ttguide.net/rear_differential_lock_solenoid_circuit_low_p17c0_-1316.html

 

All right, I will try a bit.

IF YOU ARE FOLLOWING THE TROUBLESHOOTING GUIDE...
The repair manual assumes the truck is in a full complete condition, with engine turned off, unless otherwise written. Therefore if you start the P17C0 troubleshooting procedure, you would start with the black connector plugged in, and 1) Remove the white connector from the ECU and measure the resistance from pin 1 to ground and pin 5 to ground. It doesnt matter which probe goes on Pin 1/5 and which probe goes on ground. When doing this test, first be ABSOLUTELY sure that your ohmmeter is working and reading open (typically flashing 1 or similar) correctly and reading approximately 0 ohms when probes are touched together. When doing the test on the truck make sure that you do not touch the probes with your fingers, etc which will cause a current leakage thru your body and give false results. Make sure your chosen ground is good, the cigarette lighter barrel is usually a good source of nearby ground

This test should read open - (very high) resistance from Pin 1 and Pin 5 to ground. The P17C0 code means that the ECU believes the current from the ECU going to the diff lock coil and back again is leaking out into the truck ground. If you get any resistance between P1/P5 to ground less than 10kohms, the manual indicates that the lock coil or wiring or ECU are damaged and are allowing a short circuit to ground. If that code is true, it really can only be an issue with the lock coil, the wiring between lock coil and ecu (and connectors), or the 4wd ECU

The troubleshooting manual you are reading will direct you to replace the ECU if you do not find a short circuit. If you do find low resistance, you then have to track down where the low resistance is (in the lock coil or wiring).

Some notes and personal opinions (based on anecdotal evidence): Although it is certainly possible that the ECU is bad, I have never seen a believable documented case of that on a 3rd gen tacoma, and I would put it at VERY unlikely. If you do not find a short circuit (low resistance to ground), I would NOT swap out the 4wd ECU, unless you have one handy for some reason.

If you find a short circuit, track it down... Its a lot more likely to be in the wiring (or connectors) than the lock coil, unless the rear end got cooked by an oil leak, or something happened you havent told us.

If you dont find a short circuit via ohmmeter, I would personally clear out the stored code and see if you can get it to return and use techstream to log and see if you can figure out when it is occurring (before I replaced the 4wd ECU).

Remember that this could be an intermittent short circuit, that only happens when the suspension articulates, etc.

I recommend you also get opinion from @ShimStack

 

Do what @BigWhiteTRD says. Me, I would not have even gone to the 4wd ECU and would've begun probing the connectors of the diff lock under the truck but where you are will work.

If the diff lock light has gone off I'd do the simple first and clear all codes and see what returns. If nothing returns I'd try and make it return by activating the diff lock.

 

This was very very helpful. I really appreciate it and you bearing with me. I followed your instructions and found a break in the hot wire going to the diff. locker controller. Honestly, the hardest part was wrapping my brain around resistance and continuity and reconciling that with what my multimeter was telling me. I need different one, mine doesn't allow me to change the Ohms threshold for the "beep," In this case, it was reading Open, which told me the problem was a break, not a short to ground. Again, I'm in debt to those who helped me in this thread. I learned a lot.

Here are some resources that helped me so they might help someone else searching in the future.

Diagram of differential lock circuit and testing steps:
http://www.ttguide.net/rear_differential_lock_solenoid_circuit_low_p17c0_-1316.html

Additional circuit testing steps (not for 3rd gen, but appears to be similar:
https://www.tacomaworld.com/threads/4wd-not-working.400830/page-2

Location and identification of 4wd ECU
-This is what it looks like on the 2nd gen https://www.tacomaworld.com/threads/4wd-ecu-location.495036/ (NOT SAME location NOR ECU AS 3rd gen, but it's the pic that comes up everywhere).
-on the 3rd gen, it seems to be in the roughly same place, you will need to remove multiple pieces of trim and glove box to access it. There is one long black connector and one shorter white/ clear connector. It's the only one like that in that area. This will help you ID it without having to remove it since they are stacked in tightly sideways and you can't see the part number. Note: the clear/ white plug is the one in the diagram in the trouble shooting link also mentioned above (http://www.ttguide.net/rear_differential_lock_solenoid_circuit_low_p17c0_-1316.html)



Toyota part number for harness connector (female plug end) at differential
Toyota Part No.:90980-11162
https://www.toyotapartsdeal.com/oem/toyota~housing-connector-f~90980-11162.html

Other relevant threads:
https://www.tacomaworld.com/threads/no-power-out-of-4wd-ecu.693943/
https://www.tacomaworld.com/media/img_2045.568534/
https://www.tacomaworld.com/threads...and-movement-from-front-diff-actuator.519580/

 

Glad you are getting this figured.
PLEASE PLEASE PLEASE include pictures of the damaged area, so we can direct people to check it in the future, in case this was not an isolated incident.

 

I know this is an old post but @scifidelity, did you happen to resolve the problem? do you recall what it was? having the exact same issue and haven't been able to track down the culprit.